The present invention relates generally to spectrometry, and in particular to a compact transform spectrometer which uses an intensity detector positioned in an intensity standing wave pattern produced by reflection to determine an optical spectrum.
The ability to detect light and measure its intensity is important in various fields and applications. In addition to that, it is also important to be able to determine the spectrum of the light. A variety of applications, from color cameras to advanced biological sensors would benefit from small, spectrally sensitive detection systems which integrate the ability to perform both of these functions.
At present, light detection and determination of the spectrum are performed by separate devices rather than integrated spectrally sensitive detection systems. Often, such detection systems must use some external spectrometer system because the detector element itself is not spectrally selective.
MEMS technology has enabled the miniaturization of several types of spectrometers, including Fabry-Perot interferometers, as described by P. M. Zavracky et al., xe2x80x9cA Micromachined Scanning Fabry-Perot Interferometerxe2x80x9d, Proceedings of the SPIE, 3514, 1998, pp. 179. It has also enabled the miniaturization of grating based spectrometers and Michelson Fourier-transform spectrometers as discussed by G. M. Yee et al., xe2x80x9cMiniature Spectrometers for Biochemical Analysisxe2x80x9d, Sensors and Actuators Axe2x80x94Physical, Vol. 58, 1997, pp. 61 and O. Manzardo et al., xe2x80x9cMiniaturized Time-Scanning Fourier Transform Spectrometer Based on Silicon Technologyxe2x80x9d, Optics Letters, Vol. 24, 1999, pp. 1705, respectively.
Despite the recent advances, prior art miniaturized spectrometers are still not sufficiently compact to be used for many applications. Typically, these spectrometers require beam splitters and, when used in two dimensional arrays, e.g., two dimensional arrays for collecting spectral images, they require raster scanning. Also, most of the miniature prior art spectrometers are difficult to manufacture.
Accordingly, it is a primary object of the present invention to provide a spectrometer which is compact and does not require the use of beam splitters.
It is another object of the invention to provide a compact spectrometer which can be used in two dimensional arrays for collecting spectral images without raster scanning.
Yet another object of the invention is to provide a compact transform-based spectrometer which is simple to fabricate and retains the throughput and multiplexing advantages of prior art transform spectrometers.
These and other objects and advantages will become apparent upon reading the ensuing description.
The objects and advantages set forth are achieved by a spectrometer which determines a spectrum of a light by using a mirror to reflect the light so that the light forms an intensity standing wave pattern through superposition of an incident portion of the light and a reflected portion of the light. The spectrometer is further equipped with an intensity detector whose thickness is less than a shortest wavelength in the spectrum of the light being examined. The intensity detector is also semitransparent over the spectrum. There is a mechanism for providing relative movement between the mirror and the intensity detector such that the intensity detector registers a variation of the intensity standing wave pattern. An analyzer determines the spectrum of the light from that variation of the intensity standing wave pattern.
The analyzer which determines the spectrum is a Fourier transform analyzer. The spectrum is obtained from a Fourier transform of the variation. Specifically, as the intensity detector and the mirror move with respect to one another, the amplitude of the intensity standing wave pattern varies and the Fourier transform of the resulting time domain signal determines the spectrum.
There are many possible mechanisms for providing relative movement between the intensity detector and the mirror. In one embodiment this mechanism comprises a device for moving the intensity detector. In another embodiment, the mechanism comprises a device for moving the mirror. Of course, both the mirror and the intensity detector can be moved simultaneously by different devices or by a more complex integrated mechanism. In one particular embodiment the mechanism comprises a membrane. Either the intensity detector or the mirror is mounted on the membrane. A driver is provided for oscillating the membrane to thus provide for relative movement between the intensity detector and the mirror.
The intensity detector is positioned in the path of the light such that the light passes through it first and is then incident on the mirror. In one embodiment, the intensity detector is a photoconductor deposited on a quartz wafer and the mirror is a MEMS mirror.
The invention further provides a method for determining the spectrum of light by using the variation of the intensity standing wave pattern registered by the intensity detector. The spectrum is determined from the variation by performing a Fourier transform of the variation. To simplify the transform, in a preferred embodiment the relative movement between the intensity detector and the mirror is a linear relative movement.
When the mechanism providing relative movement between the mirror and the intensity detector is an oscillating membrane it is desirable to obtain large displacement and stable motion. For this purpose the oscillation is driven substantially at a resonance of the oscillating membrane.
It is also desirable to select an intensity detector which has a substantially flat response profile of intensity versus wavelength over the spectrum. Furthermore, the mirror is preferably selected to have a substantially flat reflectivity profile over the spectrum.